The Australian Transport Safety Bureau (ATSB) is Australia's national transport safety investigator. The ATSB's function is to improve safety and public confidence in the aviation, marine and rail modes of transport. The ATSB is Australia's prime agency for the independent investigation of civil aviation, rail and maritime accidents, incidents and safety deficiencies.

Airprox - 22 km south of Williamtown, NSW

The crew of a Fairchild Industries
Ltd. SA227 Metroliner were inbound to Williamtown, NSW, when the
control zone and restricted airspace were not active. The copilot
made an inbound broadcast on the common traffic advisory frequency
(CTAF) at 20 NM south, to which no response was heard. The crew
reported that at 10 NM south of Williamtown, as they were
descending through 3,000 ft, they passed within an estimated 30 m
of a low-wing, retractable landing gear, single-engine aircraft
travelling in the opposite direction. The pilot in command, who was
the pilot flying, reported that his attempt to avoid the other
aircraft would not have been timely enough to affect separation and
there was an Airprox.

The investigation was unable to
identify the other aircraft and determine why its pilot had not
used the radio to provide positional information when operating in
the vicinity of an airport for which the use of a radio was
required and to determine if the aircraft transponder equipment (if
equipped) was serviceable.

The investigation found that since
November 2005, the provision of radar services in the Williamtown
area had been reduced when the military airspace was not active.
That was due to an unresolved technical problem with the military
secondary surveillance radar at Williamtown, which impacted the
civilian air traffic control system. A replay of the military radar
data showed that the Metroliner's radar return merged with the
primary return (no identification or altitude information) of
another aircraft travelling in the opposite direction, overhead
Newcastle.

The occurrence demonstrated the limitations of the see-and-avoid
concept as an adequate means of achieving safe separation from
other traffic in an unalerted traffic environment. It also
demonstrated the arbitrary hand played by good fortune in avoiding
a mid-air collision over a populous area, when just one pilot in an
airspace system that relies on the cooperation of all pilots,
either cannot or does not choose to participate.